Cooled insulation surface temperature control system

ABSTRACT

An apparatus and method for surface temperature control is provided. Surface temperature control is achieved by flowing coolant in and then out of a low strength porous layer attached to a structural plenum. A semi-permeable layer may be attached to the outer surface of the porous layer to prevent erosion of the porous layer and to facilitate surface film cooling.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/455,204, filed Jun. 5, 2003, the entirety of which is herebyincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is generally directed to methods and apparatusesproviding surface temperature control in a high heat flux environmentwith concurrent high velocity flow over the surface, and, moreparticularly, to surface temperature control methods and apparatusestraditionally involving film cooling or transpiration cooling in theaforementioned environment.

2. Description of the Related Art

Many engineering applications, including various components of aircraft,missiles and spacecraft, require temperature control on surfacesbounding high velocity flow while simultaneously being subjected to highincident heat flux. Conventional methods of surface temperature controlunder such conditions are film cooling and transpiration cooling.

A typical film cooling system includes a load bearing structural plenumhaving a large number of small holes drilled in the outer structuralwall thereof. The cooling air exits the plenum through these holesthereby forming a cooling film that reduces the temperature of the outerstructural wall. However, such a cooling system has disadvantages; alarge number of holes must be drilled in the surface to be cooled,increasing the cost and complexity of the plenum while reducing itsstructural strength. Furthermore, the holes must be carefully designedto give an effective cooling film over a wide variety of externalenvironments. Cooling air exiting at too high of a velocity will blowthrough and out of the surface boundary layer into the free stream flow,resulting in reduced heat transfer at the plenum outer wall andcorrespondingly poor surface temperature control.

A typical transpiration cooling system includes a plenum bounded by anouter wall consisting of a structural porous material formed from asintered metal or a ceramic. These porous materials have a large surfacearea per unit volume, and are capable of providing highly effectivecooling of the material and correspondingly good surface temperaturecontrol. However, selecting the type of porous material to use as theouter wall of the plenum is a difficult design problem. Structuralceramics tend to be brittle and have less structural strength thanmetals. Sintered metals tend to be stronger but are also heavier thanstructural ceramics, and thus may impose an unacceptable weight penalty.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a surface temperaturecontrol method is provided that includes providing pressurized airwithin a structural cooling air plenum bounded by an inner and an outerwall. A porous layer is attached to the outer wall. The porous layer mayinclude a low strength ceramic foam. Surface temperature control may beachieved by flowing cooling air into and then out of the porous layer. Asmall number of entrance holes for the coolant are provided that may bedrilled through the outer plenum wall into the porous layer.

A semi-permeable layer may be attached to the outer surface of theporous layer to prevent erosion of the porous layer and to ensure that amajority of the coolant flow exits the porous layer through small holesthat may be drilled or punched through the semi-permeable-layer. Coolantflow exiting through the surface exit holes combines with coolanttranspiring through the semi-permeable layer forming a cooling film atthe surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects, features, and advantages of the present invention will bebecome apparent upon reading the following description in conjunctionwith the drawings figures, in which:

FIG. 1 is a cross-sectional view of a cooling apparatus according to theinvention;

FIG. 2 is a plan view of the cooling apparatus of FIG. 1;

FIG. 3 is a graph of cooling effectiveness as a function of downstreamdistance that provides an example of the cooling effectiveness which canbe achieved using this invention; and

FIG. 4 is a lower surface view illustrating the surface temperature ofthe Space Shuttle following reentry, showing hot spots on the uppersurface where cooled insulation surface temperature control systems andmethods in accordance with the invention may be used.

DETAILED DESCRIPTION

With reference initially to FIG. 1, a cooling apparatus, generallyindicated at 10, includes an inner structural member 12 that is combinedwith an outer structural wall 14, forming a structural cooling airplenum 16 therebetween. The inner structural member 12 and the outerstructural wall 14 may be formed from a metallic material, such as,titanium. A porous layer 18 may be adhesively bonded or otherwiseattached to the outer structural wall 14, and entrance holes 20 may beformed in the outer structural wall 14 and may penetrate the porouslayer 18 providing flow of cooling air as indicated by the bold facearrows 22 from the plenum 16 into the porous layer 18 (for example, byproviding pressurized air within the plenum 16). The entrance holes 20may, for example, have a diameter of about 2.29 mm (90 mils), a depth ofup to one half the thickness of the porous layer 18, and may be spacedapart by about 6.9 mm (0.27″).

The porous layer 18 may have a void size of less than 50 microns, andmay be formed from a ceramic foam insulation. The low thermalconductivity of ceramic foam helps minimize the cooling required of thesurface temperature control system. The low structural strength ofceramic foam compared to conventional porous materials is unimportantsince the underlying structural plenum functions as the primary loadbearing structure. An example of the type of ceramic foam referred to isthe commercially available Rescor 360 rigid thermal insulation. Thisinsulation is manufactured by the Cotronics Corporation and may have adensity of about 256.3 kg/m³ (16 lbs/ft.³) and a thickness of about 2.54cm (1.0″). Because of the insulative qualities of the ceramic foam, itmay be bonded to the plenum using commercially available roomtemperature vulcanizing (RTV) silicone such as GE RTV-630, GE RTV-560,or Dow Corning DC3145. Bondline thickness for the adhesive may be asthin as 0.2 mm (0.008″).

A semi-permeable layer 24 may be disposed on an exterior surface of theporous layer 18. The semi-permeable layer 24 protects the underlying lowstrength porous layer from erosion by high velocity flow and may becomposed of a densification layer covered with a ceramic matrixcomposite (CMC). An example of a densification product is thecommercially available Rescor 901A liquid insulation hardener andrigidizer made by the Cotronics Corporation. Nextel 312 fabric combinedwith a sintered ceramic matrix is an example of a CMC. In an environmentwhere radiation is the dominant mode of heat transfer, thesemi-permeable layer 24 could instead be a highly reflectivesemi-permeable skin that, when bonded to the exterior surface of theporous layer 18, would both restrict transpiration and minimize absorbedenergy.

The semi-permeable barrier layer may include a plurality of perforationswhich function as exit holes 26 for the cooling air. These exit holes 26do not need to be aligned with the entrance holes 20 and may be laid outin staggered rows, forming a uniform grid as depicted in FIG. 2. Theratio of exit holes 26 to entrance holes 20 may be about 10.7 exit holesper entrance hole. The exit holes 26 may have a diameter of about 1 mm(40 mils), a depth of about 2.5 mm (0.1″) and maybe spaced 3.05 mm(0.12″) apart. The exit holes 26 may be formed using a drillingoperation or a simple and inexpensive punching operation that penetratesa portion of the semi-permeable layer 24, without the need for anexpensive drilling operation.

A heat source, indicated by arrows 30, is disposed above the coolingapparatus 10. Cooling air introduced into the plenum 16, as indicated byarrows 32, enters the porous layer 18 through the entrance holes 20, asindicated by arrows 22. The cooling air then spreads in the plane of theporous layer 18 while traveling through the thickness of the porouslayer 18, as signified by a plurality of arrows 34 shown in the porouslayer 18. The majority of the cooling air flows through the exit holes26, as indicated by arrows 36, since the semi-permeable layer 24 is asignificant hindrance to the flow of cooling air out of the porous layer18. The small amount of cooling air that does not flow through the exitholes 26 transpires through the semi-permeable layer 24 in the areasbetween the exit holes 26, as indicated by arrows 28.

The invention combines the best attributes of film and transpirationcooling while overcoming limitations in each method. This system has farfewer holes drilled through the outer wall of the plenum compared towhat would be needed in a conventional film cooling system. This makesfor a more easily manufactured and structurally stronger plenum. A smallnumber of entrance holes in the outer plenum wall maintains even surfacetemperature control because the cooling air readily diffuses bothin-plane and through the thickness of the porous layer, an effect thatis amplified by the severe restriction of transpiration that occurs atthe semi-permeable layer.

As compared to a conventional film cooling system, the coolant exitholes can be thought of as having been moved out from the outer plenumwall to the surface of the porous ceramic foam. The exit holes can beeasily manufactured in the porous foam by using a simple punch thatpenetrates the semi-permeable barrier without having to employ anexpensive drilling operation.

The ceramic foam layer additionally serves to greatly reduce the exitvelocity of the cooling air. Lower cooling velocities reduce boundarylayer penetration, thereby avoiding a common pitfall of conventionalfilm cooling systems and instead providing cooling performancecomparable to conventional transpiration cooling systems. The lowconductivity of the porous ceramic foam insulation minimizes heattransfer from the high heat flux environment and so allows the plenum tobe constructed from lower temperature, lower cost materials. The lowstrength of the lightweight ceramic foam insulation is mitigated bybonding the foam directly to the outer structural plenum wall. Thisarrangement is stronger than systems utilizing conventionaltranspiration cooled ceramics, and is lighter than porous sintered metaltranspiration cooling systems.

These benefits are realized in a system providing surface temperaturecontrol superior to conventional film cooling systems and comparable toconventional transpiration cooling systems. The thermal efficiency ofthe invention is high because the combination of film cooling andtranspiration cooling embodied in this system creates a cooling film atthe outer surface with minimal boundary layer penetration. This in turnmeans that lower coolant flow rates are needed to achieve a givensurface temperature when compared to conventional film cooling systems.

The thermal efficiency of the invention has been demonstrated inlaboratory experiments. A test was conducted in which a 2.54 cm (1″)thick porous ceramic insulation sheet with a hardened CMC semi-permeablelayer attached was adhesively bonded to a titanium substrate using hightemperature silicone. The semi-permeable layer of the sample waspenetrated by exit holes arranged in a uniform grid consisting ofstaggered rows of holes. These holes were about 1 mm (40 mils) indiameter, spaced about 3.05 mm (0.12″) apart and penetrated to a depthof about 2.54 mm (0.1″). Entrance holes having a diameter of about 2.29mm (90 mils) were drilled through the titanium substrate at a holedensity of 10.7 exit holes per entrance hole. High velocity, hightemperature air was directed tangentially over the surface of the samplewhile cooling air was blown through the sample at several flowrates.

Results from this test are presented in FIG. 3. The figure plots coolingeffectiveness η, as a function of downstream distance over the sample.Cooling effectiveness is a measure of the efficiency with which thecooling air lowers the sample surface temperature below the uncooledsurface temperature, as indicated in the equation shown annotated on theplot. A cooling effectiveness of 0.0 corresponds to a cooled walltemperature equal to the uncooled wall temperature while a coolingeffectiveness of 1.0 corresponds to a cooled wall temperature equal tothe plenum supply temperature of the coolant. Vertical lines on theplot, 38 and 40, delineate the upstream and downstream limits,respectively, of the exit hole grid on the sample surface.

The effect of two distinct cooling modes at the sample surface can beseen in the shape of the effectiveness curves. There is an initialupstream region over which the cooling film thickness buildscharacterized by rapidly rising effectiveness, followed by a fullydeveloped cooling film region characterized by approximately constanteffectiveness.

The invention achieves a high level of about 71% cooling effectivenessin the fully developed region with a modest 0.034 kg/min/cm² (0.49lbm/min/in²) cooling air flowrate. Cutting the coolant flowrate over 40%to 0.020 kg/min/cm² (0.29 lbm/min/in²) only reduces the fully developedeffectiveness to about 65%. A further reduction in flowrate to 0.013kg/min/cm² (0.18 lbm/min/in²) produces a fully developed effectivenessof about 59%. The fact that only 12 percentage points in effectivenessare lost for a nearly two thirds reduction in coolant flowratehighlights the high thermal efficiency of the invention.

The effectiveness curves also show the high degree of cooling uniformityachieved over the fully developed region of the sample surface,especially at the higher flowrates. This shows that the inventionproduces a correspondingly high degree of uniformity in surfacetemperature.

FIG. 4 depicts a vehicle in which the invention may be used, a U.S.Space Transportation System Space Shuttle orbiter 42. The orbiter 42 isboth an aircraft (i.e., during reentry and landing) and a spacecraft(i.e., during orbit). As will be readily apparent to those of ordinaryskill in the art, the cooling apparatus 10 may be incorporated intoleading edge wing structure 44 or any other structure that is subjectedto a high heat flux environment.

A cooling system according to the invention is cheaper, structurallystronger, and more thermally efficient than conventional film coolingsystems. The invention also provides a cooling system that isstructurally stronger, lighter in weight, and is at least as thermallyefficient as conventional transpiration cooling systems. Furthermore,the invention can easily be adapted to a variety of design situationsoccurring on aircraft, missiles, hypersonic vehicles, and spacecraft.

Although the preferred embodiments of the invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions, and substitutes are possible, withoutdeparting from the scope and spirit of the invention as disclosed hereinand in the accompanying claims. For example, although air has beendisclosed as a coolant, other fluids may of course be used.

1. A method of cooling a surface comprising: providing a structuralplenum bounded by an inner structural member and an outer structuralwall; providing a porous layer attached to said outer structural wallexterior to said plenum; providing a semi-permeable layer attached tosaid porous layer; providing a plurality of openings in saidsemi-permeable layer; providing openings in said outer structural wallto place said plenum in fluid communication with said porous layer; andproviding pressurized air within said plenum, wherein a majority of saidpressurized air exits said porous layer through the plurality ofopenings in said semi-permeable layer, and wherein a portion of saidpressurized air transpires through said semi-permeable layer in regionsbetween said openings, said pressurized air that exits through saidplurality of openings combines with said portion of pressurized air thattranspires through said semi-permeable layer to form a cooling filmadjacent to said surface being cooled.
 2. The method of claim 1, whereinsaid pressurized air generally flows through the thickness of saidporous layer from said openings in said outer structural wall of saidplenum to said semi-permeable layer with some spreading of flow in theplane of said porous layer.
 3. A method of cooling a surface, the methodcomprising: providing a porous layer beneath said surface; providing aplenum beneath said porous layer; providing a plurality of exit holes insaid surface; providing a semi-permeable layer in areas between saidexit holes; flowing coolant from said plenum into said porous layer andthrough said exit holes; and transpiring coolant from said plenum intosaid porous layer and through said semi-permeable layer, wherein amajority of the coolant flows through said exit holes and a minority ofcoolant transpires through said semi-permeable layer.
 4. The method ofclaim 3, wherein said plenum is formed between an inner structuralmember and an outer structural wall.
 5. The method of claim 4, whereinat least one of said inner structural member and said outer structuralwall is formed from a metallic material.
 6. The method of claim 4,wherein said porous layer is adhesively bonded to said outer structuralwall.
 7. The method of claim 6, wherein coolant flows from said plenumthrough one or more entrance holes formed in said outer structural wallinto said porous layer.
 8. The method of claim 3, wherein coolantspreads in the plane of said porous layer while traveling through thethickness of said porous layer.
 9. A method of cooling a surface, themethod comprising: providing a porous layer beneath said surface;providing a plenum beneath said porous layer; providing a plurality ofexit holes in said surface; providing a semi-permeable layer in areasbetween said exit holes; flowing coolant from said plenum into saidporous layer and through said exit holes; and transpiring coolant fromsaid plenum into said porous layer and through said semi-permeable layerwherein coolant flow exiting through said surface exit holes combineswith coolant transpiring through said semi-permeable layer forming acooling film at said surface.